We have developed a unique system for carrying out myothermal and mechanical studies on heart muscle. The central component in this system is the low thermal capacity thermopile which enables cardiac myothermalometry with previously unachieved time resolution and sensitivity. At present the system is designed for isometric studies. The value of this system will be enhanced by adding to it the capacity to make measurements on heart muscle where internal shortening is prevented during isometric measurements and controlled during isotonic measurements. We plan to use this system to study a series of cardiac hypertrophy models possessing a whole titratable range of alterations in contracility, enzymatic activity, ultrastructural properties and thermal efficiency. These range from the compensated pressure overload hypertrophy where Vmax, a/Po, myosin ATPase and mitochondrial/sarcomere volume are decreased to compensated, thyrotoxic hypertrophy where all of these paramters are increased. A whole spectrum of intermediate values have been obtained by combining the two stresses. Our strategy is to evaluate the contribution of the structural and chemical alteraiions in the hypertrophy models to their mechanical efficiencies by relating the amplitude and time course of initial heat, tension dependent heat, tension independent heat, reovery heat, and resting heat to the isometric and isotonic performance of the muscles. Measurements on control, compensated pressure overload, and compensated thyrotoxic and double treated models will enable us to further test our hypothesis (based on previously collected isometric data) that this class of muscles possess the same set of currelations between contractility, myosin ATPase, curvature of the force-velocity relation and efficiency as does the slow-fast class of skeletal muscles. Finally we plan a detailed study of the precision of our mannitol method for measuring tension independent heat to enable partitioning of initial heat into activation and contraction related components.